I'm guessing that you're truly tiny and possibly an older person to have a BMR of 1400. I'm at 1600 according to
Discovery Health's calculator and I'm 5'2" and 41 years of age. That's what I expend to lie quietly in bed doing absolutely nothing--and that's not how I spend my days. About.com's calculator assumes I need 1900 calories a day even with a sedentary lifestyle to maintain my weight.
Remember that BMR is what it takes to do absolutely
nothing. It's the energy you expend to blink, breathe, keep your heart beating, make your kidneys work, digest your food, and so forth. Cutting your caloric intake down to half of that is going to affect how well your body functions. The 1200 "rule" is really more of a guideline, but the main reason for it is that it's hard to fit in enough nutrition on a caloric intake significantly smaller than that.
A 700-calorie day is enough to live on if you're three feet tall and eighty years old. Otherwise, you will start to run into problems; JohnP already mentioned quite a few. Read up on the
Minnesota Starvation Experiment (and forgive the Wikipedia link) if you want to know more about what happens when you drop your calories too low for too long. You won't find that "starvation mode" keeps weight on you, but you will find yourself suffering what those volunteers did--and it ain't pretty.
Remember, too, that cutting calories isn't a matter of "if some is good, then more is better." I lose a pound a week on 1500 calories a day, about a 500-calorie deficit from my recommended allotment (it was a bigger deficit when I started, so my weight loss has slowed over time just because it takes less to fuel my smaller body now). If I went to 1000 calories a day, doubling my deficit, I would NOT necessarily lose two pounds a week.
It isn't that linear, and there's a floor below which the negative effects of calorie restriction outweigh the positive effects of weight loss--such as when you go bat-**** crazy from deprivation and inhale entire bags of junk food so quickly that you're picking the plastic wrapper out of your teeth. And yes, I've been there, which is how I know that I'd rather lose a pound a week and keep my sanity.
What do you currently eat to "eat properly?" Do you measure and/or weigh your food?